US 6888848 B2
A method is described for segmenting a data stream comprising variable-size packets, a data stream being defined by its source node, sink node, assigned network route, and other attributes. The segments are of equal size and the method concatenates the packets in successive segments in a manner that attempts to minimize segmentation waste without undue delay. The method facilitates the construction of efficient networks that scale to very high capacities while respecting service-quality specifications. Apparatus for implementing the method are also described.
1. A method of segmenting variable-size packets, each packet being associated with one of a plurality of defined data streams, the method comprising steps of:
concatenating packets of each data stream, from among said plurality of data streams, into concatenated packets; and
dividing each of said concatenated packets into equal-size segments,
wherein said steps of concatenating and dividing are constrained by an adaptive delay threshold;
and wherein the adaptive-delay threshold is determined by an allocated transfer rate to said each data stream;
and wherein the adaptive delay threshold is governed by a transfer rate controller.
2. The method as claimed in
3. In a network comprising source nodes and sink nodes, the method as claimed in
4. In a network comprising source nodes and sink nodes, the method as claimed in
5. In a switching node receiving packets from a plurality of incoming channels, a method of two-phase packet segmentation comprising:
segmenting individual packets in a first phase into first-phase segments using a plurality of first-phase segmentation circuits;
segmenting an aggregate of packets in a second phase into second-phase segments using a plurality of second-phase segmentation circuits;
switching an output of said first-phase segmentation circuits to said second-phase segmentation circuits under control of a routing algorithm;
wherein said routing algorithm selects routes from an incoming channel to a sink node using a method of maximum aggregation probability.
6. The method as claimed in
7. The method as claimed in
8. A data structure to enable merging equal-size null-padded data segments into new equal-size data segments having reduced null-padding, the data segments belonging to K>1 data streams, the data structure comprising:
(i) An array “A” of fractional segments having K entries, each entry dedicated to a data stream;
(ii) An array “B” of complete segments having K1>K entries; and
(iii) a control matrix “C” having K records, each record dedicated to a data stream and having four fields to indicate, for a corresponding stream, the stream's delay threshold, the stream's current delay, the fill of a respective fractional-segment in array “A”, and the address of a complete segment in array “B”.
9. The data structure as claimed in
10. The data structure as claimed in
11. The data structure as claimed in
12. In conjunction with the data structure as claimed in
(a) Receiving a plain segment of a stream k, 0≦k<K;
(b) Merging said plain segment with data content of array “A” at storage position k;
(c) Appending a complete segment resulting from said merging to a queue associated with data stream k in array “B”; and
(d) Overwriting a fractional segment resulting from said merging in memory “A” at position k.
13. The method as claimed in
14. The method as claimed in
15. In conjunction with the data structure as claimed in
(e) Receiving from a transfer rate controller a prompt to transfer a data segment of a stream k, 0≦k<K;
(f) Transferring a complete segment belonging to data stream k in array “B” if said complete segment exists; else
(g) Transferring a fractional segment belonging to data stream k in array “A” if said fractional Segment exists and has already been prompted by said rate controller a predefined number of times; else
(h) Return a no-action indication to rate controller.
16. A compact-segmentation apparatus operable to segment variable-size packets into compact fixed-size segments, the apparatus comprising
(a) a packet segmentation circuit;
(b) an enqueueing controller;
(c) a principal data memory;
(d) an auxiliary data memory;
(e) a control memory
(f) a dequeueing controller; and
(g) a transfer-rate controller;
said packet segmentation circuit receives variable-size packets and segments each of said variable-size packets into plain segments,
said packet segmentation circuit identifies a data stream associated with each one of said variable-size packets,
said enqueueing controller communicates with said control memory, said principal data memory, and said auxiliary data memory, and concatenates said plain segments of a data stream with previously-received segments of same data stream,
said transfer-rate controller selects a selected data stream, and
said dequeueiflg controller selects data segments, belonging to said selected data stream, from waiting data segments in said principal data memory and said auxiliary data memory for transfer downstream.
17. The apparatus as claimed in
18. The apparatus as claimed in
19. The apparatus as claimed in
20. The apparatus as claimed in
21. The apparatus as claimed in
22. A circuit for fast concatenation of a first picket of a known length and a second packet of a known length into a complete segment of n predefined length, the circuit comprising a shift connector, a memory array, and a register array, the length of said first packet being smeller than said predefined length and the length of said second packet being less than said predefined length, wherein
said first packet is copied onto said register array, said shift connector performs a concatenation process that concatenates said second packet with said first packet onto said register array placing any remainder resulting from said concatenation process in said memory array.
23. The circuit as claimed in
24. The circuit as claimed in
1. Technical Field
This invention relates generally to the field of data networks. In particular, it relates to a method and apparatus for segmenting concatenated variable-size packets of a data stream in order to simplify network design and increase transport efficiency while observing delay constraints to ensure high-performance.
2. Description of the Related Prior Art
A data network comprises a number of source nodes, each source node receiving traffic from numerous traffic sources, and a number of sink nodes, each sink node delivering data to numerous traffic sinks. The source nodes can be connected to the sink nodes directly or through core nodes.
The design of a data network is significantly simplified if data is transferred in fixed-size packets. However, typical data sources generate packets of variable sizes. A common approach, used in ATM for example, is to segment each packet individually into ‘cells’ of a predetermined size. A last cell of each packet would then be filled with null data. It is possible then that a large proportion of the cells become poorly utilized, depending on the packet-size distribution and the selected cell size. This can lead to relatively high capacity waste. For example, if the cell size is 1000 bits and the packet size for a high proportion of packets is within the range 1000 to 1200 bits, the relative capacity waste can be of the order of 40%. With the varying nature of data composition, it is difficult to characterize the data and to standardize a cell size that optimizes capacity utilization. Capacity waste is not only costly, but it also limits network scalability.
U.S. Pat. No. 5,930,265, issued to Duault et al on Jul. 27, 1999, describes a data processing method for efficiently transporting multimedia packets over a network that serves packets of fixed length. The method includes a step of concatenating the multimedia packets, generated by a number of users, and appending the concatenated data packets with a sub-header that identifies the individual packets. The method is primarily concerned with low-speed data and the aggregation delay is not taken into consideration.
Methods of packet concatenation that aim at minimizing capacity waste under delay constraints are required in order to realize efficient high-performance networks.
It is therefore an object of the invention to develop a method of transferring variable-size packets in fixed-size data segments without incurring a significant capacity waste or unacceptable segment-formation delay.
It is a further object of the invention to develop an apparatus for concatenating variable-size packets that belong to a common data stream.
It is another object of the invention to develop an apparatus for compact packet segmentation that can operate at incoming-channel speed.
It is a further object of the invention to develop a circuit for fast concatenating of two packets of arbitrary lengths to produce complete segments of a predefined length.
It is yet another object of the invention to develop a data segment format that promotes resource sharing by a multiplicity of users.
The invention therefore provides a method of transfer of variable-size packets, each packet being associated with a defined data stream, wherein the packets of a defined data stream are concatenated and transferred in equal-size segments. A rate controller that operates under instructions from a quality controller governs the rate of transfer of data from a source node to a sink node.
In accordance with one aspect of the present invention, there is provided, at a source node of a data network, a method of packet packing into fixed-size data segments. The packets received from incoming channels are first segmented into plain segments of a fixed size then the plain segments are merged into compact segments having less null-padding. The packing process is performed under delay constraints in order to ensure that an already received packet does not experience undue delay while awaiting merger with forthcoming packets. The method further adapts the delay limit to allocated transfer rates from a source node to a sink node.
In accordance with a further aspect of this invention, there is provided a structure of a heterogeneous data segment that enables the assembly and parsing of distinct packets. The structure includes a front header and a number of inner headers.
In accordance with another aspect of the present invention, there is provided a data structure that facilitates the formation of composite data segments, each composite data segment potentially containing data from more than one user. The data structure enables fast concatenation.
In accordance with a further aspect of the present invention there is provided an apparatus for concatenating variable-size packet under transfer-rate control and delay thresholds, the delay thresholds being determined by allocated transfer-rate control. The apparatus can concatenate packets from a multiplicity of streams. The apparatus comprises:
In accordance with a further aspect of the present invention, there is provided an algorithm for concatenating variable-size packets at incoming-channel speed where the concatenation process is performed at a rate that exceeds the rate of receiving fresh packets from the incoming channels.
In accordance with a further aspect of the present invention, there is provided a circuit for joining two fractional segments of arbitrary occupancy to produce either a single fractional or complete segment or a complete segment and a remainder. The circuit uses a shift connector for parallel word transfer. The shift connector can be constructed to arbitrarily high capacities by cascading arrays of smaller-size shift connectors.
Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In the figures which illustrate example embodiments of this invention:
It will be noted that throughout the appended drawings, like features are identified by like reference numerals.
This invention relates to a communication network transferring variable-size data packets. Packets that belong to the same data stream may be concatenated and segmented into fixed-size data blocks, a data stream being defined according to predefined attributes, such as its origin and destination. When multiple paths are provided from a point of origin to a given destination, the traffic in each path is treated as a distinct traffic stream. Packets can first be segmented, in a conventional manner, into fixed-size data blocks, hereafter also called data segments or simply segments, which may contain filler null data. A fully utilized segment is hereinafter called a “complete segment” and a null-padded incomplete segment is called a “fractional segment”.
The timing of the transfer of segments that contain portions of two or more packets must be selected so as to increase the opportunity of joining successive packets, belonging to a given stream, without resulting in excessive delays. The concatenation delay increases with the variance of packet inter-arrival time. A brute-force timing method would delay an incomplete segment until a new packet of the same stream arrives, or until a predetermined delay tolerance expires, whichever takes place first. A more efficient method, which reduces segmentation delay while attempting to maximize capacity utilization, is to use rate control in conjunction with a segmentation device. Any of prior-art rate-control mechanisms may be used.
To illustrate the need for a rate-adaptive delay threshold, consider, for example, data streams of two types, labeled “A” and “B”. A type “A” data stream has a mean bit rate of 100 kb/s (kilobits/second) and a type “B” data stream has a mean bit rate of 1 Gb/s (gigabits/second). The segment size for each type is 1 kilobit. Thus, the mean inter-segment time is 10 milliseconds for a type “A” data stream and 1 microsecond for a type “B” data stream. A typical source node would support numerous type “A” data streams and a smaller number of type “B” data streams. If a delay threshold of less than 10 milliseconds is used, the concatenation gain, i.e., the reduction in null padding, would be insignificant for the type “A” data streams. If a delay threshold of 20 milliseconds is used, which is larger than the inter-segment interval for the type “A” data streams, a reasonable concatenation gain is realized for the type “A” (and of course for the type “B”) data streams. A delay of this magnitude is, however, unacceptable for the higher speed type “B” data streams. A delay that is proportional to the segment inter-arrival time is an appropriate choice. A normalized delay threshold may be defined as an integer multiple, Q, of the inter-segment interval. A value of Q between 1 and 7, inclusive is preferred. It is noted that a value of Q equal to zero is permitted. It indicates that a segment is eligible for transfer once it is received, regardless of the extent of its null padding, if any. Thus, in general, the preferred range of Q is 0≦Q≦7.
Preferably, the network core should transfer segments in the order in which they were formed. However, if the network core can not guarantee contiguous segment succession, the segments of each stream may be consecutively labeled at respective source nodes so that the segments can be placed in proper order at the sink node. Even in a network that delivers segments in proper order, segment labeling is desirable in order to identify lost segments, if any. As mentioned earlier, a data stream is identified by, at least, a source node and a sink node. The number of bits needed to label a segment is determined by an upper bound of displacement of successive segments of the same data stream. If it can be determined that the highest displacement is, for example, less than 16 segments (a maximum of 16 interleaving out-of-order segments), then a five-bit label would suffice (to handle a disorder of +/−16 segments). In addition, conventional segment demarcation is applied.
If two segments of the same data stream are separated by T1 time slots, a time slot being a segment duration, at their source node and by T2 time slots at their sink node, then the disorder “D” is the absolute value of the difference between T2 and T1, i.e., D=|T2−T1| where |x| denotes the absolute value of a number x. The extreme disorder in a data stream is the maximum value of D experienced by any two segments in said data stream.
When multiple routes are available to carry data from a source node to a sink node, the data flow from the source node to the sink node through each route constitute a data stream.
A source node supports a multiplicity of sources and a sink node supports a multiplicity of sinks. The segments of a data stream, may belong to several source-sink pairs. Once it is ascertained that the received segments are in proper order, the sink node parses the segments according to the information in the embedded packet headers and reconstructs the packets in a manner well known in the art, then delivers the constructed packets to respective sinks. It is emphasized, however, that network architectures that guarantee segment transfer in proper order, and therefore eliminate the need for segment reordering at respective sink nodes, are known in the prior art.
The network comprises source nodes and sink nodes interconnected by core nodes. Packets of variable size are received at ingress ports of a source node. Each packet contains identifiers of its destination and an indication of its size, amongst other parameters. The packets are segmented into fixed-size plain segments of a predetermined fixed size. A plain segment contains data belonging to a single packet. The last segment of each packet may be incomplete and null padded. The plain segments are preferably packed into compact segments having a reduced padding waste. Thus, a compact segment may contain data belonging to different packets. Segment packing may take place either at the ingress ports of source nodes or at the output ports of source nodes, as will be explained with reference to
In one embodiment, the second phase segmentation is applied to the data received from segmentation circuit 430 independently. The segments formed in the first-phase and belonging to a specific incoming channel 420 are directed to a second segmentation circuit 450 where they are sorted according to a stream identifier, a stream identifier may be a sink node identifier or a sink node identifier plus a service-class identifier. The sorted segments are stored in buffers 452, each of which may contain segments destined to a specific sink node in the network. The sorted segments are intentionally delayed to join forthcoming segments of the same stream. The merged segments are then switched in a switching matrix 480A to outgoing links 490 towards respective sink nodes.
In another embodiment, the second-phase segmentation is applied to data received from a plurality of incoming channels 420. The data received from incoming channels 420 are first segmented in circuits 430 then the resulting segments are switched in a switching matrix 480B to second-phase segmentation circuits 460. Each segmentation circuit 460 handles segments belonging to a subset of the sink nodes. The segments received at a segmentation circuit 460 are carried by an outgoing link 490 towards the designated subset of sink nodes. In order to increase the second-phase segmentation gain, i.e., to minimize the null padding, the routing of data from incoming channels 420 to outgoing channels 490 should aim at selecting the paths from the plurality of incoming channels 420 to each sink node through a small number of outgoing channels 490.
The segments formed by circuit 450 are hereinafter called “combined segments”. A combined segment contains data received from the same incoming channel 420 and may belong to different traffic sources. The segments formed by circuit 460 are hereinafter called “composite segments”. A composite segment contains data that may be received from several incoming channels 420 and different traffic sources. The demarcation and parsing of composite segments will be described in more detail. Since a combined segment is a special case of a composite segment, both will be referenced as a composite segment, except when distinction is necessary.
The sizes of the segments formed in the first phase and the second phase may differ. However, second-phase segments cannot be smaller than the first phase segments. It is advantageous to select the first phase segment size to be relatively small, since the first-phase segmentation applies to individual packets and the smaller the segment, the lower the null-padding waste. Smaller first-phase segments increase the complexity of the segmentation circuit. However, this complexity is contained within the source node and the selection of the size of the second-phase segments is more critical since the latter traverse the network towards the sink node. It is preferable to select the size of the second-phase segment to be an integer multiple of the first phase segment. The second-phase segments are transported across the network and, hence, should be packed as efficiently as possible. Selecting the size of the second-phase segments to be relatively small (one kilobit for example) increases the packing efficiency at the expense of control complexity at the switching core nodes. The second-phase segment size should therefore be selected to be large enough to reduce switching-control complexity, and small enough to reduce null-padding waste. Likewise, the concatenation delay threshold should be large enough to increase the opportunity of joining packets of the same stream and small enough to meet quality-control objectives. The selection of both the second-phase segment size and the delay threshold are based on engineering judgement. A preferable second-phase segment size is 4 kilobits and the delay threshold may vary from a few microseconds to a few milliseconds, depending on the traffic stream.
It is noted that providing quality control does not necessarily mean providing a high service quality to every traffic stream. Rather the quality control function ensures that the network response is commensurate with the service expectation of each stream.
The quality-control function is governed by a quality controller, which operates to implement service-quality objectives. The quality controller may enforce providing a gauged service rate to a data stream. The gauged service rate can be set by the individual traffic sources of a data stream. The gauged service rate can also be set by a source node on behalf of its traffic sources. A source node determines an appropriate service rate for an entire data stream, which may include data from a plurality of sources, by monitoring its traffic to a plurality of sink nodes.
A second compact segmentation circuit 540 attempts to concatenate segments of a given stream, for example segments directed to the same sink node, in order to reduce the extent of null padding. The segments formed by concatenation include both plain segments and combined segments. The resulting packed segments are then switched through segment switch 511 to respective output ports 514, and thence, towards respective sink nodes 520. At a sink node 520, the segments are switched from input ports 522 to egress ports 524 through segment switch 521. At each egress port 524, a packet reconstruction circuit 550 parses the segments and reconstructs the original packets, which are then transferred to respective sinks (not shown). The segment switches 511 and 521 can be separate entities, or can be combined into a “folded” structure in a manner well known in the art. The segment switch can be implemented as a classical space switch or a rotator-based space switch as described in U.S. Pat. No. 5,745,486 issued to Beshai et al. on Apr. 28, 1998. The main advantages of the rotator-based switch are its high scalability and simplicity of contention control.
In FIG. 5 and
Switch 611 of a source node 610 and switch 621 of sink node 620, belonging to the same edge node, may handle segments of different sizes. The segment size, G1, at an input port of switch 611 is preferably smaller than the segment size, G2, at the output of aggregation circuit 640. The aggregation function in circuit 640 is easier to implement if G2 is an integer multiple of G1. The segment size at each ingress port of switch 611 is G1 and at each input port of switch 621 is G2.
In one embodiment, the two switches 611 and 621 operate independently and each can be adapted to switch segments of any specified size.
In another embodiment, the two switches 611 and 621 are combined into a single fabric and they share data memory and control. In an edge node comprising a source node and a sink node, the switching fabrics of the source node and the sink node are often combined in a single fabric. When G2 equals G1, combining a switch 611 and a switch 612 in a single fabric is straightforward. If G2 is not equal to G1, combining the two switches in a single fabric may require complex controls. However, the control of the combined single fabric is greatly simplified if the two segment sizes G1 and G2 bear a rational relationship to each other. A rotator-based switch, having a plurality of input ports and a plurality of output ports, as described in U.S. Pat. No. 5,745,486 issued to Besbai et al. on Apr. 28, 1998, enables each input port to deliver an integer number of data segments destined to one or more output ports during an input access interval and each output port to receive an integer number of data segments during an output access interval. The input access interval and the output access interval need not be equal. Furthermore, one of the inherent properties of said rotator-based switch is that the sizes of data segments at any input port need not be equal. However, to avoid capacity waste, the segment sizes are preferably predefined to favorable values. In the simplest form, with only two segment sizes G1 and G2, if G2 is an integer multiple of G1, and employing classical packing techniques, data streams adopting either segment sizes, or both, can efficiently share a common rotator-based fabric.
If the capacity per channel in network 516 is 10 Gb/s, then R3 should not exceed 10 Gb/s. The expansion in circuit 530 is expected to be high because each packet is segmented individually. The compression in circuit 540 does not completely offset the expansion in circuit 530. Typical values of the expansion in circuit 530 and the compression in circuit 540 are 1.5 and 1/1.2. This results in limiting R1 to 8 Gb/s, with N×N switching fabric 511 operating at ingress/output port speeds of 10 Gb/s. The compression ratio in circuit 640 is expected to be higher than that of circuit 540. The two circuits 540 and 640 are identical. However, circuit 640 merges packets received from several ingress ports and hence has a better gain, i.e., a better opportunity of joining other packets, than circuit 540, which merges packets from a single ingress port. It is noted, however, that the segments received at a circuit 640 may occasionally belong to a single data stream. With a value of R4 in
The network accommodates a heterogeneous mixture of connection-based and connectionless traffic. Service quality can be controlled by establishing connections, either on a per-user basis or per traffic stream, where a traffic stream may contain data from several users. Connections are traditionally established according to user-specified capacity requirement or on the basis of user-provided traffic descriptors. In order to provide a controlled service quality for connectionless traffic streams with unspecified traffic descriptors or capacity requirements, provisional connections can be introduced, as described in detail in U.S. patent application Ser. No. 09/132,465 filed Aug. 11, 1998. In general, a source node can be adapted to determine a service rate requirement for a data stream, or each of a plurality of data streams based on traffic monitoring or performance monitoring with or without consulting the traffic sources.
Each of segmentation circuits 530 and 540 are shown in
To enable the reconstruction of packets at the sink node, the segments must be labeled. Labeling is also needed for network routing if multiple hops are required to reach the sink node through network 516.
The field 712 is included to accommodate network cores that may not deliver segments in the order in which they are received. A second field 714, typically two-octets wide, indicates a connection number (CN), and a third field 716 indicates the number of immediately following data octets that belong to the connection identified by field 714.
Segments 710 in
In order to simplify the process of parsing composite segments, a predefined upper bound of the number of inner headers is preferably enforced. This, in effect, limits the number of packet memberships per segment, which would be kept below a predefined upper bound because of circuit-design considerations.
Adaptive Delay Threshold
As described earlier, applying an artificial delay to fractional segments offers an opportunity to form complete segments, and hence reduce capacity waste. However, the applied delay must be tailored to the traffic stream. In a rate-controlled network, each traffic stream is adaptively allocated a transfer rate, which is controlled by a scheduler. The delay threshold, in this case, is preferably determined as a function of the allocated transfer rate. The delay threshold may also be determined according to other criteria such as a specified service type associated with selected streams.
The transfer rate allocation to a traffic stream may be explicitly specified by a traffic source or a cluster of traffic sources, or may be determined dynamically by respective source nodes in response to monitored performance or measured traffic levels. The allocated rate, and—hence—the concatenation delay threshold, for a given stream may change with time.
A segment is ready for transfer by the dequeueing controller 870 if it is a complete segment waiting in principal data memory B or a fractional segment waiting in auxiliary data memory A and has already been prompted by the rate controller a defined number of times. The dequeueing controller 870 has a transfer queue (not illustrated) to hold segments that are selected for transfer.
Rate-controllers are well known in the prior art. U.S. Pat. No. 6,034,960, issued on Mar. 7, 2000, and titled “ATM Service scheduler using reverse-binary scattering and time-space mapping”, and U.S. Pat. No. 6,041,040, issued on Mar. 21, 2000, and titled “Large-scale service-rate regulators for ATM switching”, the specifications of which are incorporated herein by reference, describe rate controllers that can handle a large number of data streams. The described controllers apply to fixed-size data segments and are not limited to ATM applications. Either can be employed as a rate controller 880.
A service-quality controller (not illustrated) allocates a transfer rate for each data stream based on both the payload of each data stream and packet overhead. Segmenting packets into fixed-size segments incurs an additional overhead due to the added headers and the null padding. With packet concatenation in accordance with the present invention, the additional overhead can be reduced to acceptable levels by appropriate selection of the segment size and the concatenation delay thresholds. In any case, the allocation of a transfer rate for each data stream must account for any overhead.
The rate controller 880 samples the data-stream-queues represented in memory “C”. Each data stream is sampled at a nominal inter-sampling interval, which is the period between successive sampling instants of the same stream determined according to an allocated service rate for the stream. The actual sampling interval may deviate from the nominal sampling interval due to possible coincidence of the sampling instants of two or more streams. In a well-designed rate controller, the random variable representing the deviation of the actual sampling interval from the nominal sampling interval has a zero mean and a small coefficient of variation. The waiting time threshold for a given stream is conveniently expressed as an integer multiple of the nominal sampling interval of the stream.
The segments dequeued from principal data memory “B” or auxiliary data memory “A” are preferably placed in a “transfer queue” (not illustrated) within dequeueing controller 870. The use of the transfer-queue permits the dequeueing controller to simultaneously designate more than one segment as eligible for transfer.
If array 920 is shortened, as described above, and in the rare event that the shortened array is fully occupied, segments received from packet segmentation circuit 530 can wait in a buffer (not illustrated) within the enqueueing controller 830. This further reduces an already negligible probability of data loss.
As indicated above, memory “B” stores complete segments (having no null padding) that are awaiting transfer once permission is granted by the rate controller 880. Memory “A” stores fractional segments (null-padded) that are either awaiting concatenation with a forthcoming segment of the same data stream, or awaiting transfer permission despite the null-padding if a quality-control count recorded in a corresponding field 912 reaches a threshold. A reasonable threshold is two sampling intervals as determined by rate controller 880. However, the threshold may vary from one data stream to another and the threshold of each data stream is recorded in a corresponding field 911. The quality control count is triggered by rate controller 880. When the rate controller 880 prompts a stream to transfer a segment and the stream is found to have only a fractional segment in memory “A” (with no corresponding segments in memory “B”), the quality-control count in field 912 (C(1, k) for stream k) is increased by one. When the value of (C(1, k) equals the value of Q=C(0, k), the fractional segment in position k in memory “A”, structure 920, is considered ready for transfer under control of dequeueing controller 870. By limiting the length of field 912 to 3 bits, a waiting threshold of up to seven sampling intervals is permitted. Such a high delay, of seven sampling intervals, would only be suitable for data streams that are delay insensitive. The delay tolerance is stream dependent and determined by the value C(0, k), for stream k, as read from field 911.
The arrays of
Array 911 is initialized by the quality controller (not illustrated), which selects a delay threshold for each stream based on admission criteria, which are beyond the scope of this disclosure.
A fractional segment may contain data from different packets belonging to the same stream. The “age” of a fractional segment is the age of the first packet that forms the fractional segment. The algorithm shown in the flow chart of
If the number S1S2 is either “01” or “11”, the dequeueing controller 870 concludes that there is a complete segment belonging to stream k waiting in memory “B”. Selector 1114 then selects branch 1118 and control is transferred to step 1132. The existence, or otherwise, of a waiting fractional segment belonging to stream k in memory “A” is irrelevant. The complete segment is then transferred from memory “B” to the network, as indicated in step 1132, through selector 1136 and outgoing link 1140. Normal “book keeping” functions, such as the return of the address H=C(3, k) to the pool of free B-addresses, are performed in step 1134.
The processes of FIG. 10 and
FIG. 12 and
In the example of
In the example of
The segments received at a sink module 520 (
The invention therefore provides a method of, and apparatus for, packet transfer in a communications network. The data is transferred in fixed-size data segments by concatenating packets of same destination, but belonging to different users, in order to simplify the network design while minimizing segmentation waste and satisfying the service-quality requirements of each data stream.
The embodiments of the invention described above are intended to be exemplary only. Other modifications will be apparent to those skilled in the art, and the invention is, therefore, defined in the claims.